In densely populated networks, one such example including a warehouse, where it is desirable to track the location of items located in relatively close proximity, a system may include tens of thousands items to be tracked. Still further, in connection with the warehouse example, the large volume of items may be moving into and out of the warehouse with considerable frequency, where some of the units may stay in place for only a few minutes and others may stay in the same place for days or weeks. The large volume of items and the constant shuffling of the items can make it difficult to maintain an accurate fix on any one item's location within the warehouse. The same is true of other environments, where it is desirable to track the current location of large groups of items, which are within relatively close proximity.
Traditional triangulation or other relative location technologies, often generally require an ability to make good ranging measurements. This is especially problematic for a high density environment, like a warehouse, where there are a large number of surfaces off of which a signal can reflect, and/or where the item density requires a relatively high degree of precision in order to allow for the item to be readily distinguished from the other nearby items.
In absence of a realistic path loss model in an ever changing environment, power-based measurements, such as an indication of received signal strength, may not provide good ranging information. Where ranging is determined using time-based measurements, micro-second range resolution will often only enable a range resolution in the hundreds of meters, while nano-second resolution will often be required to enable a range resolution on the order of approximately one meter. However, generally, there is an inverse relationship between cost and timing resolution, and unfortunately, in environments where large number of items are to be tracked, the higher cost of higher precision timing circuits can be an important factor, and may be impractical if the higher precision timing circuits need to be associated with or incorporated into each of the items to be tracked.
Consequently, it would be beneficial to develop a cost effective method and apparatus for determining a location of a plurality of units. Still further, it would be beneficial to develop systems and methods that produce a sufficiently high location determination precision, which could be used in more crowded environments having higher item density, which largely avoids extensive usage of potentially more costly, solutions involving higher precision ranging measurements.
The present inventors have recognized that the use of neighbor lists, in conjunction with a subset of units who know or are able to determine their location may be a beneficial alternative approach, as opposed to a solution, which makes more comprehensive usage of determined ranging information. The neighbor lists could be used to establish proximity relative to one or more units that are nearby, and whose location are known. By incorporating the use of neighbor lists of nearby units, a relative proximity with respect to the reference units whose location are known may be able to be determined, which in turn can be used to provide a location estimate.